1. Field of the Invention
The present invention relates to a device for controlling a flow rate of a hydraulic operating fluid fed from a variable displacement pump such as a variable displacement swash plate type axial piston pump.
2. Description of the Related Art
A control device for a variable displacement pump has been used in an injection molding machine or a hydraulic press for performing plastic deformation working on a metal plate or the like, for example. In the injection molding machine, for example, control characteristics having high precision and high response are required in such a manner that a flow rate of a hydraulic operating fluid for driving a hydraulic piston can be controlled according to a flow rate of a molten synthetic resin while keeping a predetermined pressure for a short time of 0.2 second, for example, by the hydraulic piston.
Japanese Unexamined Utility Model Publication No. Hei 1-66483 has disclosed the typical prior art in which a swash plate acting as a variable element in a variable displacement swash plate type axial piston pump is driven by a hydraulic cylinder so that an inclination thereof is controlled, thereby controlling a discharge flow rate of a hydraulic operating fluid corresponding to the inclination and controlling a discharge pressure of the hydraulic operating fluid. In an injection process to be performed in an injection molding machine, it is necessary to cause an injection flow rate to conform to the flow of a synthetic resin while an injection pressure is being kept constant as described above. It may not be possible, however, owing to the shape of a metal mold and a synthetic resin material. In the prior art, it is very difficult to meet such requirements of the injection molding machine to stably control the inclination of the swash plate at a sufficient speed, and response is limited.
In order to solve the above-mentioned problem, there has been proposed a structure in which the flow rate of the hydraulic operating fluid discharged from the pump is kept constant and the pressure of the hydraulic operating fluid is controlled to be constant by means of an electromagnetic relief valve in order to control the pressure. Consequently, it is possible to keep response at a high speed.
With such a structure, however, a relief flow rate of the hydraulic operating fluid bled off from the electromagnetic relief valve is large. Accordingly, there is a problem in that power is wasted.
With such a structure, furthermore, if the pressure of the hydraulic operating fluid is rapidly raised, a speed of a rise in the pressure of the hydraulic operating fluid is increased more than a tilting turn speed of the pump. Consequently, there is a possibility that a high surge pressure might be generated. The pressure of the hydraulic operating fluid is detected, and the detected pressure is converted into an electric signal to control the operation of the electromagnetic relief valve. However, it is impossible to control the pressure of the hydraulic operating fluid with high precision due to a signal delay caused by the pressure detection and a signal delay caused by the conversion into the electric signal and subsequent arithmetic processings.
It is an object of the present invention to provide a control device for a variable displacement pump capable of controlling a relief flow rate obtained from an electromagnetic relief valve with high precision and hopefully capable of reducing the waste of a discharged hydraulic operating fluid as much as possible.
It is another object of the present invention to provide a control device for a variable displacement pump capable of preventing a high surge pressure from being generated.
It is further object of the present invention to provide a control device for a variable displacement pump capable of statically stabilizing the pressure of the hydraulic operating fluid fed from the variable displacement pump in a short time.
It is a further object of the present invention to provide a control device for a variable displacement pump capable of statically stabilizing the flow rate of the hydraulic operating fluid fed from the variable displacement pump in a short time.